Detection of a novel Cry2Ab toxin against Etiella zinckenella Treitschke from the Bacillus thuringiensis serovar canadensis SP142 strain

The soybean (Glycine max) is an important crop. The pod borer (Etiella zinckenella) is one of the most serious insects that attacks various Leguminosae. Common insecticidal controls are ineffective because of the insect's growth properties. Use of resistant crop varieties offers stabilisation of the yield and has benefits over the use of insecticides. Bacillus thuringiensis is widely used as a bioinsecticide for pest control and a genetic material for pest-resistant transgenic plants. However, the resistance evolution of target insects is emerging as a major threat to the long-term efficacy of these applications. Studies on the detection of novel highly host-specific pesticidal proteins have been in urgent demand. A search for the source of Bt Cry toxins against E. zinckenella in the Vietnamese B. thuringiensis strain collection has been performed. The B. thuringiensis serovar canadensis SP142 is one of strains that resulted in more than 80% mortality to this pod borer. Its genome was estimated about 7.1 Mb and revealed a putative novel cry2Ab gene. The sequence analysis of cry2Ab gene revealed an open reading frame of 1 899 bp encoding a 633-amino acid protein with a calculated molecular mass of 70 kDa and 99.05% to 99.21% homology to known cry2Ab genes in the GenBank. There are eighteen different nucleotide sites which lead to five amino acid changes in Domain I and II. This gene was expressed in Escherichia coli BL21(DE3) and the purified Cry2Ab was toxic to E. zinckenella larvae with an LC50 value of 1.74 µg/g diet. The novel Cry2Ab was designated as Cry2Ab39 by the Bacterial Pesticidal Protein Resource Center and its sequence was deposited in the GenBank (MN319700.1). This is a type of novel Cry2 toxin from B. thuringiensis against E. zinckenella, and it is important for breeding E. zinckenella-resistant soybeans.